B cells may have multiple roles in pathogenesis of CNS autoimmune disease. While data indicate that myelin-specific antibodies (Ab) promote demyelination in experimental autoimmune encephalomyelitis (EAE) and MS, the role of B cells in antigen (Ag) presentation in CNS autoimmune disease is not clear. When compared to other APC populations, B cells are efficient APC in presentation of protein Ag when they express the B cell receptor (BCR) specific for the Ag recognized by responding T cells. We hypothesize that B cells, in particular, myelin-specific B cells have an important role as APC in activation of myelin-specific T cells. We propose (1) to evaluate the contribution of myelin-specific B cells as APC in EAE and distinguish this function from the role of myelin-specific Ab in EAE. We are creating two transgenic (Tg) models, one containing B cells that express membrane MOG-specific BCR only, and one containing B cells that express membrane MOG-specific BCR and can secrete MOG-specific IgM. We will compare EAE susceptibility in these mice to B cell-deficient and MOG-specific BCR knock-in mice that can secrete all Ig isotypes. We propose (2) to examine the roles of B cell MHC II expression and myelin BCR specificity in presentation of myelin Ag in EAE. We hypothesize that B cells, in particular myelin-specific B cells, have a key role as APC in MHC II-restricted Ag presentation and activation of myelin-specific T cells in EAE and MS. (a) We will test the role of MHC II-restricted B cell Ag presentation to T cells by creating mixed bm chimera mice in which the B cell compartment is selectively deficient in MHC II expression, and compare activation of MOG-specific T cells and EAE susceptibility to bm chimera mice containing B cells that express MHC II molecules. (b) To clarify the role of BCR specificity in MHC II-restricted Ag presentation in EAE, we will examine mixed bm chimera mice in which B cells express either the MOG-specific BCR or nitrophenyl (NP)-specific BCR and do, or do not, express MHC II molecules. Recent clinical results suggest that anti-CD20 B cell depletion may be effective in MS treatment. Our preliminary data suggest that B cell depletion is beneficial in rMOG-induced EAE, but exacerbates MOG p35- 55-induced EAE. We hypothesize that the clinical benefit in rMOG-induced EAE results from reduced B cell Ag presentation or decreased secretion of myelin-specific Ab. We also hypothesize that exacerbation of MOG p35-55-induced EAE, a model that is less dependent upon B cells and Ab, represents loss of regulatory B cell function. In order to evaluate these possibilities, we will (3) examine how anti-CD20 B cell depletion influences MOG-specific T cell responses in the periphery and in the CNS in acute and chronic EAE, and examine whether MOG-specific Ab responses correlate with clinical improvement in anti-CD20 B cell-depleted mice. These studies should provide mechanistic insight regarding B cell depletion in CNS autoimmunity and address questions regarding B cell depletion that are not easily approached in MS clinical trials.